The invention relates to a method of operating an internal combustion engine with direct fuel injection.
During a starting phase of a spark-ignition internal combustion engine with direct fuel injection, large quantities of unburned fuel are produced, in particular during a cold start, since the combustion-chamber temperature of the internal combustion engine is low during a cold starting phase. Therefore adequate vaporization of the injected fuel cannot take place. On account of the low cylinder-wall temperature, some fuel proportions accumulate on the cylinder wall of the internal combustion engine and do not participate in the combustion.
U.S. Pat. No. 5,482,017 discloses a method of operating a spark-ignition internal combustion engine with direct fuel injection. In this method, during a starting phase, a fuel quantity which is greater than the requisite fuel quantity is injected during the first combustion cycle in order to compensate for the fuel proportions which have accumulated on the cylinder wall. As a result, reliable ignition of the fuel/air mixture formed is achieved. During the subsequent combustion cycles, a lean fuel/air mixture is formed in the combustion chamber and is ignited earlier in order to increase the combustion-chamber temperature or the cylinder-wall temperature. In order to heat a downstream exhaust-gas treatment device, the fuel quantity, during the subsequent combustion cycles, is then divided into an early and a late injection and a later ignition of the fuel/air mixture formed is carried out.
Patent Document WO 99/67526 discloses a method of operating a spark-ignition internal combustion engine with direct fuel injection. In this method, the pressure of the fuel injected into the combustion chamber is determined, so that the fuel is injected during the induction phase of the internal combustion engine when the fuel pressure is lower than a predetermined minimum pressure. In this case, a pressure sensor is provided, by means of which the pressure of the fuel injected into the combustion chamber is measured before the fuel enters the combustion chamber. The internal combustion engine is run in a stratified operation during the starting phase if a minimum pressure is exceeded and is run in a homogenous operation if the pressure drops below the minimum pressure.
DE 198 232 80 C1 discloses a method of operating a direct-injection internal combustion engine during the starting phase, in which method, as a function of a coolant temperature, the operation is changed over between a low-pressure start with a homogenous mixture and a high-pressure start with a stratified mixture. In this case, a high-pressure start of the internal combustion engine is initiated if the pressure in a fuel pressure accumulator has exceeded a predetermined threshold value. Here, the threshold value is stored in a characteristic map of the control device as a function of a coolant temperature.
With the methods described above, no optimum combustion is achieved during the starting operation of a spark-ignition internal combustion engine with direct fuel injection, since an operating behavior of the internal combustion engine during the starting phase without spark failure cannot be ensured.
Against this background, the object of the invention is to design the injection process during the starting phase in such a way that wetting of the combustion-chamber wall with fuel during the injection process is reduced and an ignitable mixture cloud is formed in the combustion chamber of the internal combustion engine in the vicinity of an ignition source.
This object is achieved according to the invention of operating an internal combustion engine with an injection device, wherein the method includes the steps of feeding combustion air to a combustion chamber via an inlet port, injecting fuel into the combustion chamber using a fuel nozzle arranged in the combustion chamber, igniting a formed fuel/air mixture at a certain ignition point using a spark plug arranged in the combustion chamber, and, during the starting of the internal combustion engine, selecting a high-pressure or a low-pressure start as a function of a minimum fuel pressure built up in the injection device within a defined number of cycles. The minimum fuel pressure and the number of cycles are selected as a function of a combustion-chamber temperature. The injection of the fuel into the combustion chamber takes place in a timed sequence during the starting operation.
The method according to the invention is characterized in that the minimum fuel pressure and the number of cycles during the starting of the internal combustion engine are selected as a function of a combustion-chamber temperature, the injection of the fuel into the combustion chamber preferably taking place in a timed sequence during the starting operation. Due to the timed sequence of the injected fuel quantity during a cycle, wetting of the combustion-chamber wall with the injected fuel is minimized, as a result of which the emissions, in particular the unburned fuel proportions, are reduced during the starting phase. Since a high-pressure start is not initiated until sufficient fuel pressure is present, improved atomization of the injected fuel is ensured. Furthermore, a pressure drop in the injection device during the high-pressure start on account of the high demand for fuel during the starting of the internal combustion engine is prevented by the timed sequence of the injected fuel quantity.
In a configuration of the method according to the invention, during the starting operation, the total fuel quantity injected is introduced into the combustion chamber in up to three partial quantities, i.e. the total fuel quantity can be optionally introduced into the combustion chamber in the form of one, two or three partial quantities. The timed sequence of the injected fuel quantity prevents combustion air from penetrating into the fuel injector or into the fuel distribution line, for example a common rail, during a low-pressure start on account of the low fuel system pressure. The injection during the compression stroke therefore takes place early enough irrespective of the injection strategy, so that the compression pressure does not exceed the fuel injection pressure. During the high-pressure start, due to the timed sequence of the injected fuel quantity, a homogeneous start, a mixed form consisting of a homogenous and a stratified start, or a pure stratified start can be achieved by varying the injection times.
According to a further configuration of the invention, during a low-pressure start, the up to three partial quantities are injected into the combustion chamber before the ignition point, and, during a high-pressure start, the up to two partial quantities are injected into the combustion chamber before the ignition point and one partial quantity is injected into the combustion chamber after the ignition point. Since, during the low-pressure start, the injection of the partial quantities—there may be one, two or three partial quantities—is completed before the ignition point, high HC emissions are prevented and reliable combustion of the total fuel quantity is ensured.
In a further configuration of the invention, the ignition point during the starting operation is regulated as a function of the combustion-chamber temperature and a difference between an actual speed and an idling speed. The relationship between the ignition point and the combustion-chamber temperature ensures that the combustion-chamber temperature is increased during the first cycles.
According to a further configuration of the invention, during a high-pressure start, if the fuel pressure drops below a defined minimum pressure in the injection device, the operation is changed over to the low-pressure start. This creates defined pressure conditions which permit controlled metering of fuel. This ensures that no wall wetting takes place due to the late injection of fuel.
In a further configuration of the invention, the combustion-chamber temperature is recorded by means of a temperature-measuring device at the combustion chamber or with reference to a coolant temperature of the internal combustion engine. A temperature probe is preferably attached to the cylinder wall in the region of the combustion chamber. Alternatively, a coolant temperature serves as a reference quantity for determining the combustion-chamber temperature, it being possible, in addition to or as an alternative to the coolant temperature, for the temperature of the drawn-in air mass to be used as a further reference quantity for determining the combustion-chamber temperature.
According to a further configuration of the invention, a low-pressure start is effected at a coolant temperature of less than −15° C. or greater than 90° C., a high-pressure start being effected at a minimum fuel pressure, built up in the injection device, of at least 10 bar and at a coolant temperature of between −15° C. and 90° C. At a fuel pressure of at least 10 bar, good spray quality of the injected fuel is ensured and vaporization of the injected fuel is achieved without fuel accumulating on the wall. In this case, a low-temperature start is preferred at temperatures greater than 90° C., since, due to high temperatures in the combustion chamber, sufficient fuel pressure often cannot be built up during a restarting operation of the internal combustion engine on account of the increasing gap sizes in the fuel pump. The low-pressure start preferably takes place within a temperature range of less than −30° C. or greater than 110° C., and the high-pressure start preferably takes place between −30° C. and 110° C.
Further features and feature combinations follow from the description. Practical exemplary embodiments of the invention are shown simplified in the drawings and are described in more detail below.